Facebook made changes to its ad system this week after users and political groups complained it posted “news” about online purchases users made after leaving the site. Now stories will not be published in users’ Mini-Feeds and their friends’ News Feeds unless users proactively consent, Facebook announced Friday.

In early November, Facebook launched Beacon as a new way to socially distribute information, saying it was a “core element” its online advertisement system for connecting businesses with users and targeting advertising to the audiences they want.

Charlene Li, a Forrester Research analyst, said she was surprised to find that her purchase of a table on Overstock.com was added to her News Feed, a Facebook feature that broadcasts people’s activities to their friends on the site. She said she did not see an opt-out box.

She, and others, who numbered as many as 50,000 in one online opposition group, complained that the “Big Brother” feature had crossed the line and invaded users’ privacy.

By Friday, Facebook published a statement outlining changes.

“We appreciate feedback from all Facebook users,” the statement explained. “Users now have more control over the stories that get published to their Mini-Feed and potentially to their friends’ News Feeds.”

Facebook said that “stories” about actions users take on external Web sites will continue to appear at the top of their News Feed in a way that’s easy to read, but users must click on a button marked “OK” before it is published to their friends.

“We recognize that users need to clearly understand Beacon before they first have a story published, and we will continue to refine this approach to give users choice,” Facebook explained. “If a user does nothing with the initial notification on Facebook, it will hide after some duration without a story being published.”

Users can also choose to remove the item to prevent publication. The process will repeat itself each time a user takes action on a partner site. Finally, users can access a tutorial under the “Help” link next to the story.

Facebook also sought to clarify misinformation about how Beacon works. The company said participation in Beacon is free for all partner sites, Beacon only shares specific actions on participating sites and actions are only reported through users’ Mini-Feed and friends’ News Feed.

“Facebook is not sharing user information with participating sites and never sells user information,” the company explained it its statement. “As with all its products, Facebook will continue to iterate quickly and listen to feedback from its users.”

In less than one hour 1,000 people sent the social networking site e-mail messages saying “thank you,” at the urging of one organized opposition campaign.